Process and apparatus for producing an aqueous solution containing chlorine dioxide and chlorine

ABSTRACT

A process and apparatus for producing an aqueous solution of chlorine dioxide and chlorine wherein the likelihood of autodecomposition of chlorine dioxide is significantly reduced or practically eliminated and, in the unlikely event that such CLO2 autodecomposition occurs, it is quickly detected and the process is automatically corrected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and apparatus for producingan aqueous solution containing a mixture of chlorine dioxide andchlorine. In particular, the present invention relates to a process andapparatus for producing an aqueous solution containing chlorine dioxideand chlorine wherein the chlorine dioxide and chlorine are produced bythe reaction of an alkali metal chlorate, an alkali metal chloride and amineral acid wherein autodecomposition of chlorine dioxide is detectedand controlled. Furthermore, the present invention relates to a processand apparatus for producing an aqueous solution containing chlorinedioxide and chlorine wherein the chlorine dioxide and chlorine areproduced by the reaction of an alkali metal chlorate, an alkali metalchloride and a mineral acid mixture consisting of selected ratios ofphosphoric acid to sulfuric acid in a specifically designed reactionzone so as to reduce or eliminate the likelihood of autodecomposition ofchlorine dioxide.

2. Brief Description of the Prior Art

Aqueous solutions containing a mixture of chlorine dioxide and chlorinehave found use in bleaching pulp, water treatment applications and otherend uses. Furthermore, the chlorine dioxide and chlorine values in theaqueous solution may be separated and used individually for these andother applications.

One known method for making such chlorine dioxide andchlorine-containing solutions commercially has been to react in anaqueous medium an alkali metal chlorate, an alkali metal chloride and amineral acid together to form simultaneously both desired chemicals.This known reaction is illustrated by the following reaction equation(A), wherein the preferred alkali metal chlorate has been sodiumchlorate, the preferred alkali metal chloride has been sodium chlorideand the preferred mineral acid has been sulfuric acid:

    2 NaClO.sub.3 +2 NaCl+2 H.sub.2 SO.sub.4 →2 ClO.sub.2 +Cl.sub.2 +2 Na.sub.2 SO.sub.4 +2 H.sub.2 O                            (A)

As can be seen, two moles of chlorine dioxide are produced for each moleof chlorine produced by this reaction. Sodium sulfate is also producedas a byproduct. In commercial processes, this sulfate byproduct has beeneither immediately separated from the ClO₂ and Cl₂ products or kept inthe same aqueous reaction mixture for later end use processing.

This reaction has been taught in U.S. Pat. Nos. 2,863,722 (Rapson);3,563,702 (Partridge et al.); 3,789,108 (Rapson); 3,816,077 (Fuller etal.); 4,414,193 (Fredette et al.); 4,534,952 (Rapson et al.); and inmany other U.S. Patents.

There are many known variations of this reaction. For instance, HCl maybe used instead of the alkali metal chloride For example, see U.S. Pat.Nos. 2,344,346 (Evans) and 3,933,987 (Schulz et al.). It is also knownto employ methanol instead of the alkali metal choride For example, seeU.S. Pat. No. 4,081,520 (Swindells et al.). It is also known toadditionally use a bisulfate as a reactant. For example, see U.S. Pat.No. 3,733,395 (Fuller) It is also known to make SlO₂ alone by reacting achlorate herein by referencenin their entireties. with a mineral acid inthe presence of a catalyst. For example, see U.S. Pat. No. 4,362,707(Hardee et al.). All of the above-noted U.S. Patents are incorporated

While these above noted U.S. Patents generally teach that sulfuric acidis the preferred mineral acid reactant, some references also state thathydrochloric acid or phosphoric acid may instead be used. But, in fact,only U.S. Pat. Nos. 4,362,707; 4,381,290; 4,426,463; and 4,501,824provide actual experimental data where H₃ PO₄ was used as the mineralacid instead of H₂ SO₄ in this type of reaction. See Example 8 of thosefour patents.

Moreover, some of these references make broad statements that mixturesof sulfuric acid and phosphoric acid may be employed as the mineral acidreactant for this particular reaction. For example, see column 5, lines30-33; column 6, lines 69-71 and claim 1 of U.S. Pat. No. 3,563,702(Partridge et al.); claims 1 and 5 of U.S. Pat. No. 3,816,077 (Fuller etal.); and column 3, lines 1-3 of U.S. Pat. No. 4,534,952 (Rapson etal.). But, in fact, no teaching has been found where a mixture ofsulfuric acid and phosphoric acid was ever actually used for this typeof reaction.

One problem reported with the reaction of Equation (A), above, andsimilar reactions, is chlorine dioxide "puffing" or spontaneousautodecomposition in the reaction vessel. While the reason why thisunwanted decomposition occurs is not exactly known, it is believed to betied to the reaction temperature and presence of impurities or formedbyproducts in the reaction mixture. This "puffing" is undesired becauseit causes a decrease in the yield of the ClO₂ product and, if greatenough, explosions which may damage equipment or shut down processes orboth. Also, it has been found that once this ClO₂ "puffing" begins, itmay continue to proceed unless the reaction conditions in the reactionvessel are changed.

The prior art apparatus for carrying out the reaction of equation (A),above, or similar reactions generally comprised large slendercylindrical columns (e.g. several feet high with much narrowerdiameters). The undesired ClO₂ puffing or autodecomposition wasgenerally detected by temperature monitors located in the reactionvessel and the puffing was generally corrected by passing a gas purge(e.g. air or an inert gas) through the reaction vessel. Such detectionmeans and correction means are not totally suitable for relatively smallgeneration vessels where the system must react immediately to theoccurrence of a puffing event or otherwise risk an explosion within thegeneration vessel or complete loss of ClO₂ generation or both.

Accordingly, there is a need in the art for a process and an apparatusfor producing an aqueous mixture of ClO₂ and Cl₂ by reacting an alkalimetal chlorate, alkali metal chloride and a mineral acid together wherethere is both a reduced likelihood of a chlorine dioxide "puffing" eventand, if such an undesirable event should occur, a method for quicklydetecting and automatically correcting such undesirable events. Thepresent invention does present a solution to that need.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a process for theproduction of an aqueous solution containing chlorine dioxide andchlorine wherein and controlled, which comprises:

(a) feeding to a reaction zone a first reactant stream comprising anaqueous solution containing an alkali metal chlorate and an alkali metalchloride;

(b) feeding to said reaction zone a second reactant stream comprisingsulfuric acid, or an admixture of sulfuric acid and phosphoric acid;

(c) mixing said first and second reactant streams in said reaction zone;said streams being fed into said reaction zone at rates sufficient toform a reaction product stream comprising an aqueous solution ofchlorine dioxide and chlorine in said reaction zone;

(d) transferring said resultant reaction product stream from thereaction zone to another location by educting the reaction productstream from the reaction zone by means of a vacuum generated by the flowof dilution water through an eductor;

(e) sensing the level of vacuum in said reaction zone;

(f) stopping the feeding of said second reactant stream for apredetermined time period when the level of vacuum detected in saidreaction zone falls below a predetermined control level, said stoppingof said second reactant feed stream causes said vacuum to return to apredetermined normal level; and

(g) resuming said steps (b), (c) and (d) after said predetermined timeperiod has ended.

Furthermore, the present invention is also directed to a process for theproduction of an aqueous solution containing chlorine dioxide andchlorine wherein the likelihood of autodecomposition of chlorine dioxideis minimized or eliminated, which comprises:

(a) feeding to a reaction zone a first reactant stream comprising anaqueous solution containing an alkali metal chlorate and an alkali metalchloride;

(b) feeding to said reaction zone a second reaction stream comprising anacid admixture of phosphoric acid and sulfuric acid; wherein the weightratio of said phosphoric acid to said sulfuric acid is from about 1:16to about 1:3;

(c) mixing said first and second reactant streams in said reaction zone;said streams being fed into said reaction zone at rates sufficient toform a reaction product stream comprising an aqueous solution ofchlorine dioxide and chlorine in said reaction zone; said reaction zonecomprising a packed cylindrical reaction vessel having a concave lowerportion wherein said first and second reactant streams enter and aconical upper portion where said aqueous reaction product stream exitsand wherein said reaction vessel has a ratio of internal height tointernal diameter from about 1:0.2 to about 1:1.75 and an interstitialchamber volume of about 100 to 2000 milliliters; and

(d) transferring said resultant reaction product stream from thereaction zone to another location by educting the reaction productstream from the reaction zone by means of a vacuum generated by the flowof dilution water through an eductor.

Still further, the present invention is directed to a process which is acombination of the above two process embodiments. Such a combinedembodiment provides a process for producing an aqueous solutioncontaining chlorine dioxide and chlorine wherein the likelihood ofautodecomposition of chlorine dioxide is reduced significantly orpractically eliminated, and even when a very unlikely ClO₂autodecomposition event occurs, it is easily detected and controlled.

And, moreover, the present invention is directed to an apparatus forproducing an aqueous solution containing chlorine dioxide and chlorinewherein autodecomposition of chlorine dioxide is detected andcontrolled; which comprises:

(a) storage means for a first reactant stream comprising an aqueoussolution containing an alkali metal chlorate and an alkali metalchloride;

(b) storage means for a second reactant stream comprising sulfuric acidor an admixture of sulfuric acid and phosphoric acid;

(c) a reaction zone wherein said first and second reactant streams aremixed together at rates sufficient to form a reaction product streamcomprising an aqueous solution of chlorine dioxide and chlorine in saidreaction zone;

(d) means for transferring said first reactant stream from said storagemeans (a) to said reaction zone;

(e) means for transferring said second reactant stream from storagemeans (b) to said reaction zone, said transferring means provided withmeans for stopping and starting said transfer of said second reactantstream into said reaction zone;

(f) eductor means for transferring the resultant reaction product streamfrom said reactant zone to another location by a vacuum generated by theflow of dilution water through said eductor means; and

(g) means for sensing the level of vacuum in said reaction zone; saidvacuum sensing means capable of sending an electrical output signal tostop the stopping/starting means provided in said transferring means (e)for a predetermined time period when the level of vacuum falls to orbelow a predetermined control level.

And, even moreover, the present invention is directed to an apparatusfor producing an aqueous solution of chlorine dioxide and chlorinewherein the likelihood of autodecomposition of chlorine dioxide isminimized or eliminated, which comprises:

(a) storage means for a first reactant stream comprising an aqueoussolution containing an alkali metal chlorate and an alkali metalchloride;

(b) storage means for a second reactant stream comprising an admixtureof phosphoric acid and sulfuric acid, wherein the weight ratio ofphosphoric acid to sulfuric acid is from about 1:16 to about 1:3;

(c) a reaction zone wherein said first and second reactant streams aremixed together at rates sufficient to form a reaction product streamcomprising an aqueous solution of chlorine dioxide and chlorine in saidreaction zone; said reaction zone comprising a packed cylindricalreaction vessel having a concave lower portion wherein said first andsecond reactant streams enter and a conical upper portion where saidaqueous reaction product stream exits and wherein said reaction vesselhas a ratio of internal height to internal diameter from about 1:0.2 toabout 1:1.75 and an interstitial chamber volume of about 100 to 2000milliliters;

(d) means for transferring said first reactant stream from said storagemeans (a) to said reaction zone;

(e) means for transferring said second reactant stream from said storagemeans (b) to said reaction zone; and

(f) means for transferring said resulting reaction product stream fromsaid reaction zone to another location by educting the reaction productstream from the reaction zone by means of a vacuum generated by the flowof dilution water through an eductor.

Still moreover, the present invention is directed to an apparatus whichis a combination of the above two apparatus embodiments. Such a combinedapparatus provides an apparatus for producing an aqueous solutioncontaining chlorine dioxide and chlorine wherein the likelihood ofautodecomposition of chlorine dioxide is reduced significantly orpractically eliminated and, even when a very unlikely ClO₂autodecomposition event occurs, it is easily detected and automaticallycontrolled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically one embodiment of the present processfor producing an aqueous solution containing chlorine dioxide andchlorine.

FIG. 2 illustrates a detailed cross-sectional view of the reaction zoneand water eductor shown in FIG. 1.

DETAILED DESCRIPTION

A description of the present invention will be explained through thepreferred embodiment shown in FIGS. 1 and 2. Of course, other preferredembodiments of this invention are encompassed within the broad scope ofthe present claimed invention.

Referring to FIGS. 1 and 2, an aqueous sodium chlorate/sodium chloridesolution is reacted with an admixture of phosphoric acid and sulfuricacid to form chlorine dioxide and chlorine gas in a chlorine dioxide andchlorine generation vessel 1. This reaction mixture is vacuum eductedfrom this generation vessel 1 by means of a motive water stream flowingthrough eductor 2. The resultant aqueous solution contains sufficientamounts of chlorine dioxide and chlorine to be useful for many end usessuch as waste and industrial water treament and the like.

The sodium chlorate/sodium chloride reactant solution preferablycontains from about 16.5% by weight (about 2.07 Molar) to about 35% byweight (about 4.39 Molar) sodium chlorate and about 5% by weight (about1.40 Molar) to about 25% by weight (about 5.69 Molar) sodium chloride.More preferably, the amounts of sodium chlorate and sodium chloride inthis aqueous solution are from about 20-30% by weight (about 2.51 toabout 3.76 Molar) and 10-20% by weight (about 2.28 to about 4.55 Molar),respectively. The most preferable amounts of these two reactants in thisaqueous solution is from about 22.5-27.5% by weight (about 2.82 to 3.45Molar) and about 12.5-17.5% by weight (about 2.85 to 3.99 Molar),respectively. Preferably, within these relative amounts, the molar ratioof sodium chlorate to sodium chloride is from about 1.0:0.9 to about1.0:1.15; more preferably, from about 1.0:1.03 to about 1.0:1.10.

This sodium chlorate/sodium chloride reactant solution may be obtainedfrom chlorine/caustic manufacturing plants as a byproduct.Alternatively, other alkali metal chlorates and chlorides (e.g.potassium) may be employed instead of the above-mentioned sodium salts.Another embodiment of the present invention is separately adding anaqueous chlorate solution and an aqueous chloride solution, in theresultant weight amounts and ratios mentioned above, to the reactionzone.

The phosphoric acid/sulfuric acid reactant admixture may be made fromany concentrated phosphoric acid and concentrated sulfuric acid sources.A suitable concentrated phosphoric acid solution which is commerciallyavailable is 75% by weight H₃ PO₄. A suitable concentrated sulfuric acidsolution which is commercially available is 96% by weight H₂ SO₄ (66°Be'). It is generally desirable to premix these two acids at a locationother than where the present ClO₂ and Cl₂ generating apparatus islocated for safety considerations and to better control any of the heatgenerated by the mixing operation. However, an alternative embodimentsof the present invention is to mix the two acids just prior to thereaction zone, but within the weight ratios required.

A feature of the ClO₂ autodecomposition reduction embodiment of thepresent invention, as stated above, is that active weight to weightratio of the two acids must be in the range from about 1:16 to 1:3 byweight of phosphoric acid (expressed as H₃ PO₄) to sulfuric acid(expressed as H₂ SO₄). Preferably, this weight ratio is from about 1:15to about 1:10 by weight. Most preferably, this ratio is from about 1:14to about 1:11 by weight. It has been found that these selected ratios ofphosphoric acid to sulfuric acid significantly reduce this likelihood ofClO₂ autodecomposition. However, when the above-described vacuum sensingand correction embodiment of the present invention is used without theabove-described ClO₂ autodecomposition reduction embodiment, it ispossible to employ pure H₂ SO₄ or other mixtures of H₃ PO₄ to H₂ SO₄.

At the site of the present generator apparatus, the sodiumchlorate/sodium chloride reactant solution is stored in a storage orsupply container 3 and the phosphoric acid/sulfuric acid solution isstored in storage or supply container 4. These storage containers may bemade of any suitable non-reactive material such as polyethylene orfiberglass. They should be of sufficient size to minimize the number ofrefills. It may also be desirable in some instances to provide agitationmeans and means to protect the reactants against freezing in thesestorage containers 3 and 4.

The aqueous sodium chlorate/sodium chloride reactant solution istransferred to the reaction zone 5 through chemical transfer or feedline 6 past manual shut-off valve 7 and flow rate indicator/controlleror rotometer 8 and check valve 8A. An optional pumping means (not shown)may be employed in transfer or feed line 6 to aid in the transfer of thechlorate/chloride mixture through the reaction zone. However, the vacuumcreated by eductor 2 is normally by itself sufficient to cause thischlorate/chloride solution to feed into generation vessel 1. The acidmixture is also transferred to the reaction zone 5 through chemicaltransfer or feed line 9 past manual shut-off valve 10, pumping means 30,and flow rate indicator/controller or rotometer 11 and check valve 11A.Pumping means 30 is preferably employed in transfer or feed line 9 toaid in the transfer of the acid mixture through the reaction zonebecause the density and viscosity of the acid mixture, as compared tothe chlorate/chloride solution, is much heavier and more viscous so thatthe vacuum created by eductor 2 may not be by itself sufficient to causethe acid mixture to be fed into generation vessel 1. However, in someinstances, it may be desirable to use gravity flow or other liquidtransfer techniques in transfer or feed line 9 instead of pumping means30. Preferred manual shutoff valves 7 and 10 provide the operator withmeans for overriding the automatic controls of the preferred system orwith means for stopping reactant flow when the reactant vessel isperiodically disassembled and cleaned.

The ratio of feed for each of these two reactant streams is preferablyadjusted by rotometers 8 and 11 to provide maximum yield of chlorinedioxide from the reaction zone based on the amount of sodium chlorateadded. This adjustment is done because the ClO₂ is usually a morevaluable product than Cl₂ and sodium chlorate is usually the mostexpensive reactant. Accordingly, it is desired that substantially all ofthe chlorate (i.e. at least about 90% by weight) be converted. Thepreferred volume ratio of the feed rates of the two reactant streams isapproximately 0.65 to 1.0 parts by volume of the phosphoricacid/sulfuric acid stream per 1 part by volume of sodium chlorate/sodiumchloride solution.

The mixing of the aqueous chlorate/chloride solution with the acidsolution causes an evolution of heat. Thus, this temperature of thereaction mixture in generation vessel 1 will generally be at its boilingpoint (about 60° C. to 70° C.) under the vacuum conditions created bythe motive water stream flowing through eductor 2.

The two reactant streams are each drawn into the reaction zone 5 by thevacuum created by the motive water stream through eductor 2. FIG. 2illustrates reaction zone 5 and eductor 2 in detail. The sodiumchlorate/sodium chloride solution enters reaction zone 5 through feedconduit 12 and the phosphoric acid/sulfuric acid admixture entersreaction zone through feed conduit 13. The two reactant solutions mixtogether in mixing zone 14 and, together, are drawn up through mixingconduit 15 into reaction chamber 16 which is preferably filled withpacking material 17 (e.g. one inch ceramic saddles) to insure thorough ,mixing and reaction of the reactant chemicals. As the reactants passupward through the packing 17, chlorine dioxide and chlorine gas areboth formed in the liquid reaction mixture. It should be noted that thecompletion of the ClO₂ -and Cl₂ -forming reaction does not occur in themixing zone 14, but in the reaction chamber 16. The eductor vacuumcontinues to this gas-laden liquid reaction mixture upward throughscreen 18 (which prevents the packing material from also being drawnupward) and exit conduit 19 into eductor 2, where the reaction mixtureis dissolved into the motive water stream and thus form a useful aqueouseffluent solution.

The walls 20 of generator vessel 1 as well as conduits 12, 13, 15 and 19and packing material 17 and screen 18 should all be made up ofnon-reactive materials such as high temperature-resistant plastics.Preferred plastic materials include polyvinylidene fluoride (PVDF) andfluorine-containing resins such as polytetrafluoroethylene (TEFLON).

The interior wall geometry and size of generator vessel 1 are preferablydesigned to enhance the performance of the acid mixture reactant as usedin the process of the present invention. Reaction chamber 16 ispreferably a cylinder having no corners or void spaces where reactionmixing may be minimized or ClO₂ and Cl₂ gases could be trapped oraccumulated (and thus where such trapped quantities of ClO₂ couldautodecompose or "puff"). As shown in FIG. 2, reaction chamber 16, whencylindrical, preferably has a lower or bottom portion which is roundedupward or concave, said concave portion constituting (i.e. about 1/16 toabout 1/4 of the internal height) into vertical cylindrical walls. Anupper portion (i.e. about 1/8 to 1/3 of the internal height) of thereaction chamber 16 is preferably conical in shape up to the exitconduit 19. The packing material 17 provides better reactant mixing andfurther lessens the probability of any relatively large ClO₂ or Cl₂ gasbubbles from evolving from the reaction zone 5.

The configuration of the reaction chamber 16 preferably is such that theratio of the height (i.e. measured from the top of mixing conduit 15 toscreen 18) to the diameter (i.e. widest possible diameter) is from about1:0.3 to about 1:1.75; more preferably, about 1:0.5 to about 1:1). Theusable chamber volume (i.e. interstitial volume or chamber volume afterpacking) is preferably from about 100 to 2000 milliliters, morepreferably, from about 400 to 600 milliliters. The relatively smallreaction chamber 16, as compared to the long slender columns of theprior art is sized to afford sufficient volume to substantially completethe reaction in the reaction vessel 16, but the residence time of theformed gases is minimized to further prevent or reduce autocompositionof formed ClO₂ gas.

The motive water stream in eductor 2 is supplied under pressure,preferably from about 40 to about 95 psig (more preferably from about 60to about 80 psig). This water pressure may be measured by means ofpressure gauge 21 shown in FIG. 1. The movement of this pressurizedwater stream through the venturi construction of the eductor 2 creates avacuum in reaction zone 5. This vacuum is preferably from about 550 mmHg to about 760 mm Hg, more preferably from about 600 to 730 mm Hg. Thevacuum may be measured by vacuum gauge 22 which is connected to exitconduit 19 by means of vacuum measuring conduit 23.

The aqueous reaction mixture containing the formed ClO₂ and Cl₂ afterleaving the reaction chamber 16 is dissolved in the motive water stream.Thus, the effluent water stream after the eductor 2 may contain about100 to about 2500 mg/L or more of ClO₂ and about 50 to about 1250 mg/Lof Cl₂, depending on the generation rate desired. The effluent ClO₂strength may be kept below 2500 mg/L by using a larger eductor whichuses a higher motive water flow. This may be desirable to lessen theprobability of ClO₂ autodecomposition later in the effluent stream.

This preferred process may be easily operated with an automaticelectrical control system (not shown). This control system may cause theprocess to work by opening water inlet solenoid valve 24 and thusallowing the motive water stream to pass through water feed line 25 toeductor 2. In turn, this creates the vacuum which draws the reactantsolutions through reactant feed lines 6 and 9, feedindicators/controller 8 and 11 and generator vessel 1. Where pump 30 ispresent, it is simultaneously turned on when inlet solenoid valve 24 isopened. At the end of the predetermined generation cycle or time period,the control system closes inlet solenoid valve 24 and shuts off pump 30and opens purge water solenoid valve 26 for a predetermined period oftime so that water enters generation vessel 1 through purge water line27 to purge generator vessel 1 and the effluent line. Check valves 8Aand 11A prevent the flow of purge water back into feed lines 6 and 9.

Moreover, this control system may be used to shut down this ClO₂ and Cl₂generation system if in the unlikely event that an unwanted ClO₂ puffoccurs. Since ClO₂ autodecomposition or puffing causes a drop ordecrease in the vacuum in the generator vessel 1, this vacuum decreaseis monitored by a vacuum sensing means and switch 28 connected to vacuumconduit 23. The preferred sensing means/switch ; 28 is one manufacturedby Static-O-Ring Inc. of Olathe, Kans. (Model 54PP-K118MX-J2A) Upon theregistering of vacuum decrease, the vacuum sensing means and switch 28will send an electronic signal which will immediately stop pump 30 for asufficient period of time (e.g. at least about 30 seconds, morepreferably about 1-5 minutes) for the generator vessel to purge itselfof all chlorine dioxide content. For example, under normal mostpreferred conditions the vacuum in the generation vessel 1 is run undera vacuum of about 600 to 730 mm Hg. When ClO₂ autodecomposition occurs(i.e. a puffing event), the vacuum usually immediately 1; decreases tobelow 500 mm Hg. Thus, setting the vacuum sensing means 28 to send anelectrical output signal to pump 30 when the vacuum goes to 500 mm Hg orbelow will result in an immediate stopping of the reaction in thereaction chamber 16. During this predetermined time period after adetection of a puff, the reaction chamber 16 is fed only thechlorate/chloride solution and no acid reactant. Thus, this stops anyfurther reaction and prevents the continuation of undesired puffing.After this predetermined time period for process adjustment, the pump 30is started up again to allow acid flow. A solenoid valve may be usedinstead of pump 30 where such pumping means are not needed to aid thefeeding of the acid mixture into the generation vessel 1.

Another preferred embodiment of this invention encompasses theintroduction of a chemical reactant into either exit conduit 19, waterstream 25, eductor 2 or the effluent stream which selectively reactswith and removes the Cl₂ in the reaction mixture or effluent stream andthus leaves an aqueous solution of ClO₂ substantially free of Cl₂. Suchaqueous solutions of substantially pure ClO₂ have particular use intreating potable water streams. One chemical reactant which reacts muchfaster with Cl₂ than ClO₂ and, thus, may be useful herein is hydrogen

Advantages of these combined process embodiments of the presentinvention include the following:

1. The overall size of the generator (excluding the reactant storagetanks) is relatively small. For example, it may be mounted on afree-standing support backing wall having an area of six feet by sixfeet or less. Furthermore, the overall weight of the generator isrelatively light. The weight of the support and the generator (againexcluding the reactant storage tanks) may be less than 250 pounds.Together these relatively small size and weight characteristics makethis apparatus easily transferable or usable in crowded spaces.

2. There is no need for external heating or cooling means to thegenerator vessel. Thus, energy costs are minimized and no additionalheating or cooling equipment is needed as required in prior artprocesses.

3. All of the spent reactants are discharged through the eductor. Thereis no buildup of any reactants or byproducts in the generator vessel.Therefore, the generator vessel does not have to be frequently shut downand cleaned.

4. The combination of phosphoric acid and sulfuric acid in theabove-noted weight ratios significantly reduces the possibility ofunwanted "puffing" or the spontaneous autodecomposition of chlorinedioxide. Further, the internal geometry and size of the generationvessel further reduces the possibility of this unwanted "puffing". Thus,high stoichiometric yields of ClO₂ may be achieved on a continuousbasis. Moreover, even if the puffing event should occur, the design ofthe preferred generating apparatus allows for the immediate stoppage ofpuffing and return to the desired generation mode. All of these featuresincrease the safety and reliability of the whole system. Thus, thisgenerator may be run in out-of-the-way places without having an operatorcontinuously monitoring its operation.

5. The ClO₂ and Cl₂ generation rates may be easily varied over a widerange.

6. Inexpensive materials of construction may be used for constructingthe generator compared to titanium material employed on prior artsystems.

7. Because of the relatively small size of the generator, the reactiontime is relatively fast (e.g. less than 2 minutes). Moreover, theprocess may be quickly started up and shut down. This gives the operatorgood control over the process being treated.

8. The process of this invention enables using NaClO₃ for small-scaleproduction of ClO₂. Heretofore, there were no chlorate-based processeswhich lent themselves to small-scale production of ClO₂.

The following Examples and Experiments are provided to better illustratethe present invention. All parts and percentages are by weight unlessexplicitly stated otherwise.

EXAMPLES

A chlorine dioxide and chlorine generation system as shown in FIGS. 1and 2 was experimentally employed to control microbiological organismsin an industrial cooling tower. The aqueous sodium chlorate/sodiumchloride reaction solution contained 25.5% by weight sodium chlorate and14.9% by weight sodium chloride. The mixed acid reactant solutioncontained 7.14% by weight H₃ PO₄ [supplied by concentrated solution ofphosphoric acid (75% strength)] and 86.86% by weight H₂ SO₄ [suppliedfrom 96% concentrated sulfuric acid (66° Be')]. In Table 1, fourdifferent runs with different reactant feed rates are shown. The motivewater pressure was measured by pressure gauge 21 and water flow raterefers to the water entering the eductor 2 through motive water feedline 25 as shown in the FIGURES. The vacuum amounts listed were thevacuum in the generator vessel as measured on vacuum gauge 22. Thetheoretical chlorine dioxide amount is based on 100% conversion ofsodium chlorate into ClO₂ by reaction equation (A) above. The actualchlorine dioxide amounts were those ' amounts measured by means of aHach Spectrophotometer Model DR/3000 employing Procedure Codes C.5 orC.6. Cl₂ was not measured in these Examples.

                                      TABLE I                                     __________________________________________________________________________                                               ClO.sub.2                                                                           ClO.sub.2                    Chlorate Feed                                                                         Volume Ratio                                                                          Acid Feed                                                                           Motive Water                                                                          Motive Water                                                                          Vacuum                                                                             Theoretical                                                                         Actual                                                                             % Actual                cc/Min. Chorate to Acid                                                                       cc/Min.                                                                             Pressure psi                                                                          Flow Rate gpm                                                                         mm Hg                                                                              mg/L  mg/L %                       __________________________________________________________________________                                                          Theoretical             33.32   1:0.90  30.00 75      9.9     715  202   194  96                      75.00   1:0.80  60.00 65      9.8     695  460   445  97                      135.00  1:0.66  90.00 65      9.8     695  827   790  96                      265.00  1.0.66  175.00                                                                              65      9.8     693  1623  1550 96                      __________________________________________________________________________

Experiments 1-11

Various admixtures of concentrated phosphoric acid.sup. ○1 andconcentrated sulfuric acid.sup. ○2 (See Table II for each particularweight/weight ratio of these acids) were combined with an aqueoussolution containing 3.2 molar sodium chlorate and 3.4 molar sodiumchloride in an apparatus similar to that shown in FIGS. 1 and 2. Thetemperature of each reactant solution before mixing was room temperature(25° C.).

The feed ratio for these reactant solutions are given in Table II foreach experiment. As can be seen from Table II, the feed rates differedfor each experiment. These differences were to determine generatoroutput over different operating conditions. It should also be noted thata large excess of acid was used for each of these experiments. An acidexcess will accelerate the likelihood of puffing. Thus, in theseexperiments, an attempt toward puffing was induced. The mixing of theacid mixture with the aqueous solution of chlorate/chloride produced anexotherm and the temperature of each combined reaction mixture rose toabout 60°-70° C. as measured by a thermocouple attached to the mixingzone below the packed ClO₂ and Cl₂ generation vessel. The combinedreaction mixture was pulled upward through the saddle packing of thegeneration vessel by means of a vacuum created by a water eductor streamconcurrently running at the top of the generation vessel. The amount ofeductor water for each experiment was six gallons per minute. The vacuumcreated by the eductor stream was from about 700 to about 500 mm Hg foreach experiment and was measured by means of a vacuum gauge as shown inFIG. 1.

Besides the H₃ PO₄ /H₂ SO₄ weight ratio and the feed rates for the tworeactant streams, Table II below shows the Time to Decomposition (inseconds) and Average Percentage Yield. Time to Decomposition is the timefrom the start of the mixing of the reactants until an unwanted"puffing" event was observed. Such events were shown by a drop or lossin the vacuum in the generator vessel as seen on the vacuum gauge. Thelonger the time before decomposition, the more suitable the reactionmixture would be in a commercial process. The average percentage yieldis based on the amount of sodium chlorate reactant employed One mole ofsodium chlorate should theoretically produce one mole of '' chlorinedioxide and one-half mole of chlorine. When looking at yields alone, thehigher the yields, the would be.

The amount of chlorine dioxide (in ppm) was

in the eductor water stream by means of a Hach Spectrophotometer ModelDR/3000 employing either Procedure Codes C.5 or C.6. The ppm of Cl₂ weredetermined from the educted water stream by the amperometric titrationtechnique described by Aieta et al., J.A.W.W.A., January 1984, p. 64.

The yields of ClO₂ and Cl₂ were then calculated from these measured ppmvalues and the amount of sodium chlorate reactant.

                                      TABLE II                                    __________________________________________________________________________    Effects of Different H.sub.3 PO.sub.4 :H.sub.2 SO.sub.4                       Ratios on ClO.sub.2                                                           Decomposition and ClO.sub.2  and Cl.sub.2  Yields                             H.sub.3 PO.sub.4 :H.sub.2 SO.sub.4                                                                     Chlorate/Chloride   Yields                           Weight/Weight Acid Mixture                                                                             Stream Feed Rate                                                                        Time to   ClO.sub.2  Cl.sub.2              Experiment                                                                          Ratio   Feed Rate (ml/min)                                                                       (ml/min)  Decomposition (sec)                                                                     ppm   %    ppm %                 __________________________________________________________________________    1     0:1     192        76        30        644   92-95%                                                                             N.M..sup.4                                                                        N.M.              2     1:60    192        76        30        N.M.  N.M. N.M.                                                                              N.M.              3     1:30    192        76        45        N.M.  N.M. N.M.                                                                              N.M.              4     1:15    90         40        >240      299   93-96%                                                                             N.M.                                                                              N.M.              5     1:7.5   112        52        >240      416   84%  N.M.                                                                              N.M.              6     1:3     128        48        >300      384   84%  N.M.                                                                              N.M.              7     1:0     225        48        N.D..sup.3                                                                              <50   <10% N.M.                                                                              N.M.              8     1:13.50 21         18        >240      86    71%  34  53                9     1:13.50 45         23        >240      114   81%  54  80                10    1:13.50 45         45        >240      212   83%  101 76                11    1:13.50 37         41        >240      244   100% 120 96                __________________________________________________________________________     .sup.3 N.D. = Not Determinable. This experiment did not show any evidence     of ClO.sub.2  decomposition, however, this may be due to the fact that        that almost no ClO.sub.2  was formed.                                         .sup.4 N.M. = Not Measured. The yield of ClO.sub.2  or Cl.sub.2  were not     measured for these experiments.                                          

The times to decomposition>240 and>300 in Table II mean that nodecomposition occurred during the time period of these experiments. Thedata in Table II indicate that increasing the phosphoric acid tosulfuric acid ratio reduces the probability of ClO₂ decomposition. Thedata also indicate range of H₃ PO₄ :H₂ SO₄ (weight to weight) ratioswhich provide superior yields with lower probability of ClO₂decomposition are from about 1:16 to about 1:3 (weight to weight). Thedata also show that about 1:15 to about a 1:10 (weight to weight) H₃ P₄:H₂ SO₄ ratio provides the more optimum characteristics.

What is claimed:
 1. A process for the production of an aqueous solutioncontaining chlorine dioxide and chlorine, which comprises:(a) feeding toa reaction zone a first reactant stream comprising an aqueous solutioncontaining an alkali metal chlorate and an alkali metal chloride; (b)feeding to said reaction zone a second reactant stream comprisingsulfuric acid, or an admixture of sulfuric acid and phosphoric acid; (c)mixing said first and second reactant streams in said reaction zone;said streams being fed into said reaction zone at rates sufficient toform a reaction product stream comprising an aqueous solution ofchlorine dioxide and chlorine in said reaction zone; (d) transferringsaid resultant reaction product stream from the reaction zone to anotherlocation by educting the reaction product stream from the reaction zoneby means of a vacuum generated by the flow of dilution water through aneductor; (e) sensing the level of vacuum in said reaction zone; (f)stopping the feeding of said second reactant stream for a predeterminedtime period when the level of vacuum detected in said reaction zonefalls below a predetermined control level indicative of ClO₂autodecomposition, said stopping of said second reactant feed streamcauses said vacuum to return to the above predetermined control level;and (g) resuming said steps (b), (c) and (d) after said predeterminedtime period has ended.
 2. The process of claim 1 wherein said normalvacuum level in said reaction zone is from about 550 mm Hg to about 760mm Hg and step (f) occurs when the sensed vacuum control level is about500 mm Hg or below.
 3. The process of claim 2 wherein said predeterminedtime period in step (f) is at least about 30 seconds.
 4. The process ofclaim 3 wherein said predetermined time period in step (f) is from about1 minute to about 5 minutes.
 5. A process for the production of anaqueous solution containing chlorine dioxide and chlorine, whichcomprises:(a) feeding to a reaction zone a first reactant streamcomprising an aqueous solution containing an alkali metal chlorate andan alkali metal chloride; (b) feeding to said reaction zone a secondreaction stream comprising an acid admixture of phosphoric acid andsulfuric acid; wherein the weight ratio of said phosphoric acid to saidsulfuric acid is from about 1:16 to about 1:3; (c) mixing said first andsecond reactant streams in said reaction zone; said streams being fedinto said reaction zone at rates sufficient to form a reaction productstream comprising an aqueous solution of chlorine dioxide and chlorinein said reaction zone; said reaction zone comprising a packedcylindrical reaction vessel having a concave lower portion wherein saidfirst and second reactant streams enter and a conical upper portionwhere said aqueous reaction product stream exits and wherein saidreaction vessel has a ratio of internal height to internal diameter fromabout 1:0.5 to about 1:1.75 and an interstitial chamber volume of about100 to 2000 milliliters; and (d) transferring said resultant reactionproduct stream, from the reaction zone to another location by eductingthe reaction product stream from the reaction zone by means of a vacuumgenerated by the flow of dilution water through an eductor.
 6. Theprocess of claim 5 wherein said reaction vessel has a ratio of internalheight to internal diameter from about 1:0.5 to about 1:1.
 7. Theprocess of claim 5 wherein said reaction vessel has an interstitialvolume of about 400 to about 600 milliliters.
 8. A process for theproduction of an aqueous solution containing chlorine dioxide andchlorine, which comprises:(a) feeding to a reaction zone a firstreactant stream comprising an aqueous solution containing an alkalimetal chlorate and an alkali metal chloride; (b) feeding to saidreaction zone a second stream comprising an acid admixture of phosphoricacid and sulfuric acid, wherein the weight ratio of said phosphoric acidto said sulfuric acid is from about 1:16 to about 1:3; (c) mixing saidfirst and second reactant streams in said reaction zone; said streamsbeing fed into said reaction zone at rates sufficient to form a reactionproduct stream comprising an aqueous solution of chlorine dioxide andchlorine in said reaction zone, and wherein said reaction zonecomprising a packed cylindrical reaction vessel having a concave lowerportion wherein said first and second reactant streams enter and aconical upper portion where said aqueous reaction product stream exitsand wherein said reaction vessel has a ratio of internal height, tointernal diameter from about 1:0.2 to 1:1.75 and an interstitial chambervolume of about 100 to 2000 milliliters; (d) transferring the resultantreaction product stream from the reaction zone to another location byeducting the reaction product stream from the reaction zone by means ofa vacuum generated by the flow of dilution water through an eductor; (e)sensing the level of vacuum in said reaction zone; (f) stopping thefeeding of said second reactant stream for a predetermined time periodwhen the level of vacuum detected in said reaction zone falls below apredetermined control level, indicative of ClO₂ autodecomposition saidstopping of said second reactant feed stream causes said vacuum toreturn to the above predetermined control level; and (g) resuming saidsteps (b), (c) and (d) after said predetermined time period has ended.9. The process of claim 8 wherein said normal vacuum level in saidreaction zone is from about 600 mm Hg to about 730 mm Hg and step (f)occurs when the sensed control vacuum level is about 500 mm Hg or below.10. The process of claim 8 wherein said predetermined time period instep (f) is at least about 30 seconds.
 11. The process of claim 8wherein said predetermined time period in step (f) is from about 1minute to about 5 minutes.
 12. The process of claim 8 wherein saidreaction vessel has a ratio of internal height to internal diameter ofabout 1:0.5 to about 1:1.
 13. The process of claim 8 wherein saidreaction vessel has an interstitial volume of about 400 to about 600milliliters.
 14. An apparatus for producing an aqueous solutioncontaining chlorine dioxide and chlorine, which comprises:(a) storagemeans for a first reactant stream comprising an aqueous solutioncontaining an alkali metal chlorate and an alkali metal chloride; (b)storage means for a second reactant stream comprising sulfuric acid, oran admixture of sulfuric acid and phosphoric acid; (c) a reaction zonewherein said first and second reactant streams are mixed together atrates sufficient to form a reaction product stream comprising an aqueoussolution of chlorine dioxide and chlorine in said reaction zone; (d)means for transferring said first reactant stream from said storagemeans (a) to said reaction zone; (e) means for transferring said secondreactant stream from said storage means (b) to said reaction zone, saidtransferring means provided with a means for stopping and starting saidtransfer of said second reactant stream into said reaction zone; (f)eductor means for transferring the resultant reaction product streamfrom said reaction zone to another location by a vacuum generated by theflow of dilution water through said eductor; and (g) means for sensingthe level of vacuum in said reaction zone; said vacuum sensing meanscapable of sending an electrical output signal to stop thestopping/starting means provided in said means (e) for a predeterminedtime period when the level of vacuum falls to or below a predeterminedcontrol level indicative of ClO₂ autodecomposition.
 15. The apparatus ofclaim 14 wherein said vacuum sensing means sends the electrical outputsignal to stopping/starting means provided in means (e) when a vacuum ator below about 500 mm Hg is sensed.
 16. An apparatus for producing anaqueous solution containing chlorine dioxide and chlorine, whichcomprises:(a) storage means for a first reactant stream comprising anaqueous solution containing an alkali metal chlorate and an alkali metalchloride; (b) storage means for a second reactant stream comprising anadmixture of phosphoric acid and sulfuric acid, where the weight ratioof phosphoric acid to sulfuric acid is from about 1:16 to about 1:3; (c)a reaction zone wherein said first and second reactant streams are mixedtogether at rates sufficient to form a reaction product streamcomprising an aqueous solution of chlorine dioxide and chlorine in saidreaction zone; said reaction zone comprising a packed cylindricalreaction vessel having a concave lower portion wherein said first andsecond reactant streams enter and a conical upper portion where saidaqueous reaction product stream exits and wherein said reaction vesselhas a ratio of internal height to internal diameter from about 1:0.5 toabout 1:1.75 and an interstitial chamber volume of about 100 to about2000 milliliters; (d) means for transferring said first reactant streamfrom said storage means (a) to said reaction zone; (e) means fortransferring said second reactant stream from said second storage means(b) to said reaction zone; and (f) means for transferring said resultingreaction product stream from said reaction zone to another location byeducting said reaction product stream from said reaction zone by meansof a vacuum generated by the flow of dilution water through an eductor.17. The apparatus of claim 16 wherein said reaction vessel is packedwith ceramic saddles.
 18. The apparatus of claim 16 wherein saidreaction vessel has a ratio of internal height to internal diameter ofabout 1:0.5 to about 1:1.
 19. The apparatus of claim 16 wherein saidreaction vessel has an interstitial volume of about 400 to about 600milliliters.
 20. An apparatus for producing an aqueous solutioncontaining chlorine dioxide and chlorine, which comprises:(a) storagemeans for a first reactant stream comprising an aqueous solutioncontaining an alkali metal chlorate and an alkali metal chloride; (b)storage means for a second reactant stream comprising an admixture ofphosphoric acid and sulfuric acid, wherein the weight ratio of saidphosphoric acid to said sulfuric acid is from about 1:16 to about 1:3;(c) a reaction zone wherein said first and second reactant streams aremixed together at a rate sufficient to form a reaction product streamcomprising an aqueous solution of chlorine dioxide and chlorine in saidreaction zone; said reaction zone comprising a packed cylindricalreaction vessel having a concave lower portion wherein said first andsecond reactant streams enter and a conical upper portion where saidaqueous reaction product stream exits and wherein said reaction vesselhas a ratio of internal height to; internal diameter from about 1:0.2 toabout 1:1.75 and a interstitial chamber volume of about 100 to about2000 milliliters; (d) means for transferring said first reactant streamfrom said storage means (a) to said reaction zone; (e) means fortransferring said second reactant stream from said storage means (b) tosaid reaction zone, said transferring means provided with means forstopping and starting the transfer of said second reactant stream; (f)eductor means for transferring the resultant reaction product streamfrom said reaction zone to another location by a vacuum generated by theflow of dilution water through said eductor means; and (g) means forsensing the level of vacuum in said reaction zone; said vacuum measuringmeans capable of sending an electrical output signal to stop thestopping/starting means in means (e) for a predetermined time periodwhen the level of vacuum falls to or below a predetermined control levelindicative of ClO₂ autodecomposition.